Residual current devices (RCD) are known to serve as protective means in electrical systems in the event of contact between the bodies of electrical operating materials, and they are provided to offer protection in special areas of electrical systems.
The mode of operation of such a residual current device (RCD) relies on the fact that when an electrical system is working perfectly, the vectorial sum of the currents on all current-carrying conductors of a supply line is equal to zero, and thus there is no magnetic field around the supply line. If a residual current does occur and is drained outside of the supply line via an object or the ground due to an insulation fault, a differential current results. The variable magnetic field of this differential current induces a current on the secondary side, which triggers a switching element to isolate the faulty supply line.
In its simplest, most basic variant, based on the principle of induction a residual current device (RCD) is only able to detect temporal fluctuations in the magnetic flux, and accordingly in practice only pure AC residual currents or AC differential currents. However, consumers that are connected to the electrical system, for example electric machines comprising electronic semiconductor components such as diodes or thyristors in rectifiers or frequency inverters, are also capable of generating residual currents with a curve that is not purely sinusoidal, but instead have a pulsing plot over time. Consequently, residual current devices (RCD) have been designed that are able to detect these pulsed DC fault currents as well as purely AC fault currents. Such pulse-current sensitive residual current devices (RCD) are also called type A residual current devices (RCD).
The properties for (fixed) residual current devices of type A are defined in the standards IEC 61008 and 61009, and in standard IEC 62335 for portable residual current devices (PRCD) for various rated differential currents (rated release currents). However, the protective effect of type A residual current devices (RCD) is only operative if they are selected correctly with regard to rated differential currents and the influence of DC fault currents.
In practical operation, it has proven disadvantageous that the release behaviour of such devices, that is to say the dependency of response value and response time, can be affected negatively by currents and frequencies that are above the specified ratings. In this context, the core of the measuring current transformer may be pre-magnetised depending on the magnitude of the DC fault current and/or the magnitude and frequency of the AC fault current, and this pre-magnetisation raises the release threshold, or even prevents the RCD from being released altogether.
Such a situation may occur for example while an electric vehicle is being charged at a charging station if the charging station provides AC voltage and is equipped with a type A residual current device (RCD). If an insulation fault in the charging installation causes the DC fault current—determined essentially by the insulation resistance of the combined unit of charging station and electric vehicle—to exceed a maximum permissible DC fault current limit of 5 mA or 6 mA, as defined in the technical standards, the function of the type A residual current device (RCD) in the charging station may be restricted.
Accordingly, DC fault currents above a value of 6 mA, the maximum permitted for type A residual current devices (RCD), are considered critical. With regard to AC fault currents, values higher than 30 mA for example and a frequency of 50/60 Hz or above 1000 Hz are considered critical for type A RCDs. These standardised limits for maximum permissible fault currents in type A residual current devices (RCD) may vary from country to country. For example, the maximum permissible DC fault current in the USA is 5 mA, whereas the currently applicable standard IEC value is 6 mA.